Why does this material change colors throughout the animation?
The reason why the tunnel gets brighter and darker is because of the point lights that you've scattered across the tunnel. Areas closer to the lights (orange dots shown below) are brighter.



You can fix that by moving the lights with the camera instead of fixing the lights into a single spot. Delete all but two of the lights and parent them to the camera (yellow dot). It doesn't really move the lights perfectly along the middle of the tunnel, but you could always animate them manually to fix that.